Thumbnail images (or, briefly, “thumbnails”) are small images derived from larger images by sub-sampling.
Generation of thumbnails is an ordinary feature of personal computers and a desired feature of other consumer electronic (CE) devices such as digital photo frames, digital cameras, mobile phones, multi-function printers, etc. These devices are equipped with a screen or other display unit allowing the user to preview and/or browse digital photos and digital pictures. Providing high-quality and short-time-consuming image thumbnail generation is quite helpful in these such devices. Digital photo previewing and browsing are exemplary of the applications involving decoding a digital image (typically in JPEG format) and resizing it in order to be displayed on a specific target screen, while taking into account quality and processing time constraints.
Generation of thumbnails is addressed extensively in the scientific and technical literature including the patent literature. WO-A-94/22108, US-A-2006/242163, and U.S. Pat. Nos. 6,263,119 and 6,778,707, which are incorporated by reference, are exemplary of patent documents related to thumbnail generation.
So far, two basic approaches have been resorted to for thumbnail generation.
A first approach involves a sub-sampling procedure in the image pixel domain. After choosing a zoom-out factor, based upon the relative dimensions of the target screen and the original image, data is filtered to obtain the desired resolution. To that end, resizing is performed through a line-by-line scanning, once the original image has been completely decoded (i.e., brought back to the image pixel domain), which inherently requires a considerable amount of computational time.
Another approach involves resolution scaling implemented using spatial frequency subsampling in the DCT or transform domain, during the image decoding step. For instance, a 8×8 DCT block can be sub-sampled using a scaling ratio selected out of {1, 1/2, 1/4, 1/8} thus filtering out the high frequencies. Smaller scaling ratios permit significantly faster decoding since fewer coefficients need to be processed and a simpler IDCT method can be used. For instance, when choosing a ratio equal to 1/8, only the DC component is considered. The remainder of the decoding process, including the de-quantization of the quantized AC coefficients and the IDCT, can be “skipped”. Resizing may thus become a very fast process as the number of coefficients to be processed decreases, while, however, the final result may fail to optimally fit to the target screen and its shape (e.g., aspect ratio).
Additionally, certain systems such as certain embedded systems may not have sufficient memory resources to perform thumbnail caching as conventionally described in the literature, namely thumbnail caching into system mass storage (e.g., hard disk) or removable media (e.g., USB disk, SD card), which in any case involves writing files into the file system of the storage device. When removable mass-storage is used, if the media is abruptly removed during the write stage, file system corruption may occur.